ELECTRICAL CIRCUITRY FOR USE e.g. IN FORCE BALANCE SERVOSYSTEMS

ABSTRACT

The invention is concerned with electrical circuitry for ensuring that the mean current through a load, e.g. a force balance winding for a pressure transducer, bears a substantially linear relationship to the mark/space ratio of pulse width modulated signals whose mark/space ratio varies e.g. with an error signal developed at a pickoff of the pressure transducer. The circuitry comprises an integrator, a load, e.g. the aforementioned winding, a current-sensing resistor in series with the load, a switch which connects the load to the said circuit means or to circuit earth, a source of the aforementioned p.w.m. signals, means for operating the switch to connect the load to the said circuit means or to circuit earth accordingly as the p.w.m. signals are at one voltage level or the other, and a comparator which receives the p.w.m. signals and the voltage at the junction between the load and the current-sensing resistor and which supplies to the integrator a voltage which represents the difference between the p.w.m. signals and the junction voltage. The time constant of the integrator is chosen in relation to the cycle time of the p.w.m. signals so as to ensure that variation in load current, arising, e.g. as a result of temperature variations, gives rise to a voltage change at the output of the integrator such change ensuring that the mean load current bears the aforestated relationship to the mark/space ratio of the p.w.m. signals.

United. States Patent [72] Inventors Peter Michael Knight;

Michael John Toou, both of Kent, England [2i] Appl. No. 864,411 [22]Filed Oct. 7, 1969 [45] Patented Dec. 14, 1971 [73] Assignee ElliottBrothers (London) Llmlted London, England 1 [3 2] Priority Oct. 9, 1968[3 3] Great Britain [3] 47,813/68 [54] ELECTRICAL ClRCUl'lRY FOR USEE.G. lN

FORCE BALANCE SERVOSYSTEMS 4 Claims, 5 Drawing Figs.

[52] U.S. Cl 318/676, 3 l 8/599 [5l] lnt.Cl ..G05b 11/01 [50] Field ofSearch 3 l8/599, 676

[56] References Cited UNITED STATES PATENTS 2,985,808 5/l96l Ketchledge..3 l 8/20.290 UX 3,213,694 10/1965 Clark etal. 3l8/32X PrimaryExaminer-T. E. Lynch Anorney-Kirschstein, Kirschstein, Ottinger & FrankABSTRACT: The invention is concerned with electrical circuitry forensuring that the mean current through a load, e.g. a force balancewinding for a pressure transducer, bears a substantially linearrelationship to the mark/space ratio of pulse width modulated signalswhose mark/space ratio varies e.g. with an error signal developed at apickoff of the pressure transducer.

The circuitry comprises an integrator, a load, e.g. the aforementionedwinding, a current-sensing resistor in series with the load, a switchwhich connects the load to the said circuit means or to circuit earth, asource of the aforementioned p.w m. signals, means for operating theswitch to connect the load to the said circuit means or to circuit earthaccordingly as the p.w.m. signals are at one voltage level or the other,and a comparator which receives the p.w.m. signals and the voltage atthe junction between the load and the current-sensing resistor and whichsupplies to the integrator a voltage which represents the differencebetween the p.w.m. signals and the junction voltage.

The time constant of the integrator is chosen in relation to the cycletime of the p.w.m. signals so as to ensure that variation in loadcurrent, arising, e.g. as a result of temperature variations, gives riseto a voltage change at the output of the integrator such change ensuringthat the mean load current bears the aforestated relationship to themark/space ratio of the p.w.m. signals.

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INVENTORS PETER M. KNIGHT MICHAEL T. TOOZE BY 2b i k m g ATTQ RNEYSELECTRICAL CIRCUITRY FOR USE E.G. IN FORCE BALANCE SERVOSYSTEMS Thisinvention relates to electrical circuitry for use, e.g. in

force balance systems.

Circuitry according to the invention comprises:

circuit means operable to develop an output voltage which is the timeintegral of a variable voltage with respect to a certain datum voltage;

a load;

current-sensing means in series with the load;

a switch which, in one position, connects the load to the output of thesaid circuit means and, in another position, connects the load to asource of reference potential e.g. circuit earth;

a source of pulse width modulated (p.w.m.) signals;

means for operating the switch so as to connect the load to the outputof the circuit means or the said source of reference voltage accordinglyas the p.w.m. signals are at one voltage level or another; and

comparator means to which are applied the pulse width modulated signalsand the voltage developed at the junction between the load and thecurrent-sensing means, and which supplies to the input of the saidcircuit means, a voltage representing the difference between the p.w.m.signals and the junction voltage; and

the time constant of the circuit means is chosen in relation to thecycle time of the p.w.m. signals so that variations in load current dueto drift or other variations in load characteristics, e.g. as a resultof temperature variations, give rise to a change in voltage at theoutput of said circuit means, so as to ensure that the mean load currentbears a substantially linear relationship to the mark/space ratio of thep.w.m. signals.

The source of p.w.m. signals may operate between predetermined referencevoltage levels. I

In an hereinafter described embodiment the source of p.w.m. signalscomprises:

another switch connected to the comparator;

second and third reference voltage sources, one of which may be circuitearth; and

a source of primary p.w.m. signals which is operative to connect thesaid other switch to the second or third reference voltage sourceaccordingly as the primary p.w.m. signals are at onevoltage or another,and which is operative, also, to operate the first mentioned switch soas to connect the load to the output of the said circuit means or thefirst mentioned source of reference voltage.

According to the invention, also, a force balance system comprises:circuitry as specified in the last preceding paragraph; a force balancetransducer which is to be maintained against a reaction, at a nullposition as represented by the development of a null signal at theoutput of the transducer pickofi', by current flow in an electricalwinding which constitutes the load for the said circuitry; and a circuitarrangement, constituting the said source of primary p.w.m. signalswhich are controlled as to mark/space ratio by error signals developedat the pickoff output.

The circuit means may comprise: an integrator the output of which isconnected to the first mentioned switch and the input of which isconnected to the comparator output.

The circuit means may comprise: an integrator whose output is connectedto the comparator output; a voltage source operative to develop asubstantially constant datum voltage; and voltage summing means whichreceives the datum voltage and the output voltage from the integratorand whose output is connected to the first-mentioned switch.

The invention is hereinafter described with reference to theaccompanying drawings in which:

FIG. 1 is a block schematic diagram of a force balance pressuretransducer system for an aircraft;

FIG. 2' depicts circuitry employed in producing controlled current flowin an electrical winding of the pressure trans ducer;

FIG. 3 is a general block schematic circuit diagram of a source ofprimary p.w.m. signals;

FIG. 4 is a more specific block schematic circuit diagram of a source ofprimary p.w.m. signals, and

FIG. 5 depicts waveforms utilized in the force balance system.

The system (FIG. 1) comprises: a force balance pressure transducer,static or pitot-static, 11 which has an electrical winding 13 and an ACpickoff 15; there is a source of primary pulse width modulated signals17 which receives error signals developed at the pickoff output; andelectrical circuitry 19, which may be considered to include the winding13, receives pulse width modulated signals from the source 17 anddevelops in the winding 13 a DC current, the mean value of which bears alinear relationship to the mark/space ratio of the p.w.m. signals.

The force balance pressure transducer may be as described in U.K. Patentspecification No. l,2 l 5,14 l.

The circuitry 19 (FIG. 2) comprises circuit means 21 operable to developan output voltage V which is the time integral of a variable voltagewhich, in practice, is applied to the input of the circuit means, withrespect to a certain datum voltage V,,,; the winding l3 of the forcebalance transducer; current sensing means, in the form of a resistor 23,in series with the winding 13; a switch 25 which, in one position,connects the winding 13 to the output of the circuit means 21 and, inanother position, connects the winding 13 to a source of referencevoltage which, as shown, may be circuit earth; a source (27 and FIG. 4)of p.w.m. signals S; means, 29 and FIG. 4, for operating the switch 25so as to connect the winding I3 to the output of the circuit means 21 orto circuit earth accordingly as the p.w.m. signals are at one voltagelevel or another; and comparator means 31 to which are applied thep.w.m. signals S and the voltage V developed at the junction 33 betweenthe winding 13 and the resistor 23 and which applies to the input of thecircuit means 21 a voltage representing the difference between thep.w.m. signals S and the junction voltage V,-,,.

The time constant T of the circuit means 21 is chosen in relation to thecycle time of the p.w.m. signals S so that variations in thecharacteristics of the winding 13 due to drift or other variations, e.g.as a result of temperature changes, give rise to a change in thevoltageV at the output of the circuit means 21, so as to ensure that themean current in the winding 13, bears a substantially linearrelationship to the mark/space ratio A of the p.w.m. signals S.

The circuit means 21 may,,as shown, comprise an integrator 35 a source37 of signals m, and a summing point 39. The voltage source 37 and thesumming point 39 may, however, be omitted and the datum'voltage wouldthen be zero. In the absence of the voltage V,,. the integrator wouldhave to integrate up to the mean voltage V,, from its zero datum.

The source 27 of p.w.m. signals 8 comprises the source 17 (FIGS. 2 and4) of primary p.w.m. signals P; reference voltage sources V and earthand another switch 41. The switch 41 connects the comparator 31 to thereference voltage V or to earth accordingly as the primary p.w.m.signals from the source 39 are at one voltage level or another. Thep.w.m. Signals S supplied to the comparator therefore have themark/space ratio of the primary p.w.m. signals and a voltage whichstands at one or the other of two precisely defined values, V or earth.

The source 17 of primary p.w.m. signals is also connected to theactuating means 29 of the switch so that the switch 25 is operatedsynchronously with the switch 41.

In operation, the operation of the switch 25 by the primary p.w.m.signals P applies to the winding 13 a p.w.m. waveform whose mark/spaceratio A is equal to that of the primary p.w.m. waveform and whosevoltage stands at V, the output voltage of the circuit means 21, or atearth potential. The current flowing in the winding 13 in response tothe p.w.m. voltage applied by the switch has a mean value which bears alinear relationship to the mark/space ratio of the p.w.m.

signals. As the mark/space ratio varies so does the mean current in thewinding 13.

The characteristics of the winding 13 are liable to variation as aresult e.g. of temperature changes. Such variations are reflected as achange in mean load current and, hence, in a change in voltage V at thejunction 33. The latter point is connected to an input of the comparator31.

In steady state conditions, i.e. when there is no change incharacteristics, there is no net output from the comparator and thevoltage V from the circuit means 21 is unchanging. When, however, thevoltage V changes, e.g. as a result of a temperature change, adifierence signal is supplied from the comparator to the integrator ofthe circuit means 21. The result is a change in V, the output voltage ofthe circuit means. The change in V compensates for the change incharacteristics of the winding, with the result that the mean loadcurrent level remains substantially unaffected by such changes.

In broad terms the source 17 of primary p.w.m. signals comprises (FIG.3): a demodulator 45, an error voltage sign threshold detector 47, and adigital p.w.m. generator 46 which receives signals from the thresholddetector 47 on one or the other of two inputs accordingly as the p.w.m.waveform is to be increased or decreased as to mark/space ratio.

Typically, the source 17 (FIG. 4) of primary p.w.m. signals maycomprise: an amplifier 43, demodulator 45, a voltage thresholdarrangement 47 which develops an output signal on one 49 or the other 51of two inputs accordingly as an analogue signal supplied from thedemodulator 45 exceeds certain limits; an add/subtract digital counter53; gating circuitry 54 controlled by clock pulses C (FIG. from a clockpulse source 55 so as to supply to the add/subtract counter digitalsignals, derived signals appearing at the outputs of the thresholdarrangement 47, so as to increment or decrement the content of thecounter; a register 57, logic circuitry 59 controlled by recurrentsignals A (FIG. 5) from a pulse generator 61 so as to periodicallyregister in the register 57 the content of the counter 53 and aflip-flop 63 which is switched to one state by the recurrent signals Aand reset by a pulse appearing at the output of a gate 65 when thecontent of the register 57 is reduced to zero by recurrent signals B(FIG. 5) from the pulse generator 61.

An analysis of the circuitry of FIG. 2 follows.

In the analysis:

V =reference voltage )t==mark/space ratio of input p.w.m. waveformT==time constant of integrator V=integrator output voltage i=current inwinding 13 R=resistance of winding I3 L=inductance of winding 13R,,=resistance of current sensing resistor Note: r=average value of iaveraged over a few cycles of p.w.m. frequency.

Hence it can be seen from (3) that steady state -r=- A and from (4) thatif -r is disturbed it returns to the true value of -r with Also, it canbe seen from (5) that steady state V=EZEQ R which is independent of A.

Thus as A varies the integrator output V will not be required to changeonce V has attained its correct value.

This means, therefore, that the average current 1' through the winding13 will always directly follow )t.

The only reason for V changing subsequently is to correct for changes inR and L arising from temperature variation which will be comparativelyslow and hence the value of T can be chosen suitably.

We claim:

1. Electrical circuitry which comprises:

circuit means operable to develop an output voltage which is the timeintegral of a variable voltage with respect to a certain datum voltage;

a load;

current-sensing means in series with the load;

a switch which, in one position. connects the load to the output of thesaid circuit means and, in another position. connects the load to asource of reference potential e.g. circuit earth;

a source of pulse width modulated (p.w.m.) signals;

means for operating the switch so as to connect the load to the outputof the circuit means or the said source of reference voltage accordinglyas the p.w.m. signals are at one voltage level or another; and

comparator means to which are applied the pulse width modulated signalsand the voltage developed at the junction between the load and thecurrent sensing means, and which supplies to the input of the saidcircuit means, a voltage representing the difference between the p.w.m.signals and the junction voltage; in which the circuit means comprises:an integrator whose output is connected to the comparator output; avoltage source operative to develop a substantially constant datumvoltage; and voltage summing means which receives the datum voltage andthe output voltage from the integrator and whose output is connected tothe first mentioned switch; and in which the time constant of thecircuit means is chosen in relation to the cycle time of the p.w.m.signals so that variations in load current due to drift or othervariations in load characteristics, e.g. as a result of temperaturevariations, give rise to a change in voltage at the output of the saidcircuit means, so as to ensure that the mean load current bears asubstantially linear relationship to the mark/space ratio of the p.w.m.signals.

2. Circuitry according to claim I in which the source of p.w.m. signalsoperates between predetermined reference voltage levels.

3. Circuitry according to claim 2 in which the source of p.w.m. signalscomprises:

another switch connected to the comparator;

second and third reference voltage sources, one of which may he circuitearth; and

maintained against a reaction, at a null position as represented by thedevelopment of a null signal at the output of the transducer pickoff. bycurrent flow in an electrical winding which constitutes the load for thesaid circuitry; and a circuit arrangement, constituting the said sourceofprimary p.w.m. signals which are controlled as to mark/space ratio byerror signals developed at the pickofi output.

1. Electrical circuitry which comprises: circuit means operable todevelop an output voltage which is the time integral of a variablevoltage with respect to a certain datum voltage; a load; current-sensingmeans in series with the load; a switch which, in one position, connectsthe load to the output of the said circuit means and, in anotherposition, connects the load to a source of reference potential e.g.circuit earth; a source of pulse width modulated (p.w.m.) signals; meansfor operating the switch so as to connect the load to the output of thecircuit means or the said source of reference voltage accordingly as thep.w.m. signals are at one voltage level or another; and comparator meansto which are applied the pulse width modulated signals and the voltagedeveloped at the junction between the load and the current sensingmeans, and which supplies to the input of the said circuit means, avoltage representing the difference between the p.w.m. signals and thejunction voltage; in which the circuit means comprises: an integratorwhose output is connected to the comparator output; a voltage sourceoperative to develop a substantially constant datum voltage; and voltagesumming means which receives the datum voltage and the output voltagefrom the integrator and whose output is connected to the first mentionedswitch; and in which the time constant of the circuit means is chosen inrelation to the cycle time of the p.w.m. signals so that variations inload current due to drift or other variations in load characteristics,e.g. as a result of temperature variations, give rise to a change involtage at the output of the said circuit means, so as to ensure thatthe mean load current bears a substantially linear relationship to themark/space ratio of the p.w.m. signals.
 2. Circuitry according to claim1 in which the source of p.w.m. signals operates between predeterminedreference voltage levels.
 3. Circuitry according to claim 2 in which thesource of p.w.m. signals comprises: another switch connected to thecomparator; second and third reference voltage sources, one of which maybe circuit earth; and a source of primary p.w.m. signals which isoperative to connect the said other switch to the second or thirdreference voltage source accordingly as the primary p.w.m. signals areat one voltage or another, and which is operative, also, to operate thefirst mentioned switch so as to connect the load to the output of thesaid circuit means or the first mentioned source of reference voltage.4. A force balance system which comprises: circuitry as claimed in claim3; a force balance transducer which is to be maintained against areaction, at a null position as represented by the development of a nullsignal at the output of the transducer pickoff, by current flow in anelectrical winding which constitutes the load for the said circuitry;and a circuit arrangement, constituting the said source of primaryp.w.m. signals which are controlled as to mark/space ratio by errorsignals developed at the pickoff output.